A vehicle head-up display apparatus includes a display device that emits display light from a display panel, such as a liquid crystal panel.
In such a display device, a light source device that emits non-polarized illumination light is typically used as the light source device for the illumination light irradiated onto the display panel. Examples of light source devices that emit non-polarized illumination light include light emitting diodes (LEDs). Only a particular polarized component of the illumination light irradiated onto the display panel is transmitted by a polarization filter provided on the light source device side of the display panel. Consequently, other polarized components in the illumination light cannot pass through the display panel, leaving room for improvement in the use efficiency of the illumination light.
A composite optical element, for example, that includes a first mirror and a second mirror that face each other, a functional film arranged between the mirrors at a certain angle, a retardation plate adjacent to the first mirror, and a light source device between the retardation plate and the functional film has been developed to address this issue. For example, see patent literature (PTL) 1. Such a composite optical element causes a certain polarized component (s-wave) within the non-polarized illumination light emitted by the light source device to be reflected by the functional film and emitted towards the display panel and causes another polarized component (p-wave) to be transmitted by the functional film to reach the second mirror. The p-wave reflected at the second mirror is converted to circularly polarized light upon passing through the retardation plate, is reflected by the first mirror, passes through the retardation plate again to become an s-wave, and is reflected by the functional film to be emitted towards the display panel. The use efficiency of the illumination light has been improved in this way.